In fields such as a statistical decision theory, artificial intelligence, machine learning, data mining and the like, a decision tree is used for building of an expected model, decision-making analysis/optimization, resolution of a classification problem, description of concept/knowledge, rule extraction/generation and the like. The decision tree is represented with a tree diagram by hierarchizing repetitions of branches. As a technique for automatically generating a decision tree, a technique for automatically generating a decision tree based on, for example, an average amount of information (entropy) of each case in a case set.    Patent Document 1: Japanese Laid-open Patent Publication No. HEI9-330224